PROJECT SUMMARY Attending to meaningful sounds in noisy environments challenges individuals with hearing impairment. In animal models of auditory attention, the response of neurons in auditory cortex are altered to enhance the discriminability of stimuli relevant to current behavioral demands. However, it has proven difficult to disentangle effects of attention to particular acoustic features from nonspecific arousal. I will use a combination of neurophysiology and pupil measurement in awake ferrets to compare the influence of arousal and task engagement in primary auditory cortex (A1). Building on previous research that has demonstrated correlations between spontaneous changes in pupil size and sensory coding in mouse visual and auditory cortex, I will first examine the statistical relationship between pupil size and: (1) responses to natural sounds in A1, (2) patterns of variability that are shared across neurons (noise correlations) and (3) neural frequency selectivity. I will then examine changes in response properties while ferrets perform a tone-versus-noise discrimination task. By comparing the response of the same neurons to identical stimuli across pupil sizes, I will measure arousal- related changes in gain, baseline firing rate, correlated variability, and frequency tuning. By comparing neural activity across behavior and passive listening, I will measure the effect of task engagement on the same variables. I hypothesize that arousal will alter response properties such as gain and baseline firing rate, which do not specifically affect the representation of task-relevant stimuli, while task engagement will shape frequency tuning in a task-specific manner. In addition to exploring the neural mechanisms of auditory attention, this research will offer insight into the auditory neural correlates of pupil size, an easily-accessible measurement of changes in brain state that has been proposed as a clinical measurement of listening effort.